severed head that needs oxygen?

it's definitely a stretch, even in a goofy horror movie that's supposed to be about a real scientific breakthrough formula that re-animates dead.

There would be no breathing
It wouldn't be able to talk
The body should not have functioned , the whole science was that the formula re-animates the brain, allowing other functions to happen , but obviously a severed head has no connection.
Some have said that he had mind powers sort of established in a deleted scene but you can't mind control something with no mind lol

It's an artifact from the original story in which West hypothesizes that there is a psychic link between mind and body.

You are sin.

I took it as Hill was so traumatized that he would breath just out of habit.

The brain needs oxygen or else it will die. Hence why people get brain damage sometimes when they nearly drown or overdose on drugs and stop breathing. Parts of the brain begin to die from lack of oxygen, so once the person is revived after a period of time of not breathing they can often suffer permanent brain damage. The head would still require lungs and a circulatory system/heart though so the blood can be pumped by the heart through the circulatory system to bring the oxygenated blood to the brain, supply the brain with oxygen, then be pumped to get more oxygen from the lungs, and repeat until something stops the process. Other than my basic understanding of how oxygen is moved through the human body, I don’t have any other answers to your other questions.

The people who made the movie didn't give two shits about biological science if it got in their way. The severed head had lines of dialogue in the script, so it needed to talk, so it needed to breathe, so it did. There is no possible explanation of how a severed head could breathe or talk if it's not connected to any sort of lungs, but the people who made the movie simply didn't care. If you can't get past that, that's your problem.

What? This movie had something unrealistic in it?